1. Field of the Invention
This invention relates generally to display screens and, more particularly, to a method and apparatus for controlling the brightness of a display screen.
2. Description of the Related Art
Light concentrating diode displays, which are commonly known as LCD displays, have been used for a number of years in a wide variety of applications. LCD displays are probably most commonly used for small digital readouts, such as the digital displays found in watches and calculators. The area of such LCD displays is typically no larger than one square inch. As most people who own a watch or calculator having an LCD display are aware, LCD displays must be illuminated to be viewed in dim lighting. Accordingly, most devices having relatively small LCD displays include a light bulb that effectively illuminates the display so that the user of the device can read the display in dimly lit environments.
LCD displays have become increasingly larger in size and, thus, are being used in applications much more complex than providing a simple digital readout. For example, LCD displays are currently a popular choice for desktop computers, portable computers, personal information organizers, and the like. The area of these relatively large displays is typically greater than five square inches, and these displays may be larger than 100 square inches. Because of their relatively large size, these displays cannot be effectively illuminated using only one light bulb positioned to shine on or behind the display. Rather, such a technique, which was used with early large LCD displays, creates a bright portion, sometimes referred to as a "hot spot," on the display. The bright portion may be so bright as to "wash out" any characters or information contained on that portion of the display, while the remaining portion of the display may be too dark to read.
To solve this problem, elongated light bulbs have been developed in conjunction with diffusion screens. In a typical arrangement used for AC powered devices, a first elongated light bulb is placed at the top of an LCD display and a second elongated light bulb is placed at the bottom of the LCD display. A diffusion screen is placed behind the LCD display between the first and second light bulbs. This type of diffusion screen is a single sheet of plastic that has reflectors formed in it. These reflectors get progressively larger toward the center of the diffusion screen. To illuminate the LCD display, both light bulbs are illuminated, and the light from each bulb is diffused by the diffusion screen to provide a relatively consistent brightness level over the entire area of the LCD display.
By contrast, in a typical arrangement used for DC and AC/DC powered devices, a single elongated light bulb is placed at the top of an LCD display, and a diffusion screen is placed behind the LCD display below the light bulb. This type of diffusion screen is also a single sheet of plastic that has reflectors formed in it. However, unlike the previously discussed diffusion screen, these reflectors get progressively larger toward the bottom of the diffusion screen. To illuminate the LCD display, the light bulb is illuminated, and the light from the single bulb is diffused by the diffusion screen to provide a relatively consistent brightness level over the entire area of the LCD display.
It should be understood that single light bulbs are used in DC and AC/DC powered devices to conserve battery power when the LCD display is being illuminated, because a single light bulb consumes less power than two light bulbs. However, as compared with the LCD displays used in AC powered devices that use two light bulbs, the LCD displays used in DC and AC/DC powered devices suffer from the reduced brightness levels produced by the single light bulb. This disadvantage remains even if an AC/DC powered device is operating on AC power, because the device contains only a single light bulb.
The present invention is directed to addressing one or more of the matters set forth above.